I found that even where I can parse a technical text (understand all introduced notions, without needing to look up the notions that are used without being defined), it's not a sufficient condition for me being ready for the text. It takes a lot of background effort to build technical fluency that allows to take away a deeper and lasting understanding of a given topic, fluency that isn't required to merely parse the text, or even solve the exercises and ace the exam. Without this fluency, without being prepared, acquired knowledge remains superficial, never becomes very useful, and quickly fades out of memory.

It's like reading a novel in a barely known foreign tongue, translating with a dictionary, and juggling the syntax without feeling the flow of the language. Technically, you can translate everything, but there is no hope for understanding the subtle points of the narrative, and the only way to get there is through obtaining fluency first, and reading the novel later.

What this tells me is that where I can't even parse a text on my own (i.e. there is a non-negligible number of statements I can't understand, or exercises I don't see how to solve), this is an absolutely unambiguous indicator that I'm not ready to try this particular text, and should work on something more elementary.

(This is a strategy for building deep knowledge of a favored subject; it's much more useful to skim in order to obtain superficial general knowledge of many diverse subjects, although elementary textbooks should still be the way to go, not recent research papers.)